Wells to wheels: Environmental implications of natural gas as a transportation fuel☆
Introduction
In 2015, natural gas (NG) was the most produced energy source in the U.S., accounting for 32% of the total U.S. energy production (U.S. Energy Information Administration, 2016a). NG production in the U.S. has increased from 23.5 trillion cubic feet (TCF) in 2006 to 32.9 TCF in 2015 (U.S. Energy Information Administration, 2016b). This is largely due to production of NG from shale formations through the advent of horizontal drilling and high-volume hydraulic fracturing technologies over the past decade, which increased from 2.0 TCF in 2007 to 15.5 TCF in 2015, accounting for 47% of U.S. natural gas production currently (U.S. Energy Information Administration, 2016b). The shale gas boom is primarily a U.S. phenomenon as Canada and China are the only other countries that are producing commercial volumes of shale gas (U.S. Energy Information Administration, 2016c). A large portion of the U.S. shale gas production growth has occurred in the Marcellus play, where production reached an average of 5.7 TCF in 2015 (U.S. Energy Information Administration, 2016d), as shown in Fig. 1. Fig. B.1 shows the geographic distribution of major shale plays in the U.S. (U.S. Energy Information Administration, 2016e).
Abundant gas reserves and increased production provides an opportunity to expand the use of NG in various end-use sectors in the U.S. In 2015, NG is the second most consumed energy source (29%) in the U.S., of which 35% was consumed in the electric power sector, followed by the industrial sector (33%), residential and commercial sector (28%), and the transportation sector (3%). The transportation sector has been explored as a growth sector for NG, as it consumes predominantly petroleum-based fuels and accounts for 28% of the total primary energy consumption in the U.S. (U.S. Energy Information Administration, 2016f). As the NG cost has been consistently low lately, significant research and development has been undertaken to transform it to fuels, such as Fischer-Tropsch diesel, dimethyl ether, and hydrogen. In addition, significant efforts have been made to overcome technical and market barriers of its use as compressed natural gas (CNG) and liquefied natural gas (LNG). Research needs for impact analysis and assessment of expanding the use of NG fuels on energy consumption, emissions, energy security, and ownership costs have been highlighted (Wang et al., 2015a).
The objective of this study is to perform a wells-to-wheels (WTW) analysis of freshwater consumption, GHG emissions, air pollutant emissions, and ownership costs of CNG and LNG vehicles in the heavy-duty vehicle (HDV) sector. The WTW system boundary includes the wells-to-pump (WTP), which includes the entire NG supply chain that involves with production, processing, transmission, storage, and distribution of NG to CNG or LNG plants, as well as the pump-to-wheels (PTW) including refueling and use of CNG or LNG fuels by vehicles. This study provides important results on the environmental, technological, and economic impacts of introducing NGVs in the U.S. heavy-duty vehicle sector, which represents the second largest and fastest growing share of transportation energy demand (U.S. Energy Information Administration, 2016g). In this analysis, we examined three key NGV markets: refuse trucks, transit buses, and freight trucks (TIAX, 2013), and two types of NG fuels, i.e., CNG and LNG.
The process of hydraulic fracturing (HF, also known as “fracking”)” uses large quantities of high pressure fluid to create fractures in a rock formation thousands of feet below the earth's surface and facilitate the release of shale gas and liquid (EPA, 2016). More than 90% of the HF fluid volume is water, and the remaining portion usually consists of proppant (2–10% of the total volume, mostly sand) and additive chemicals (U.S. Environmental Protection Agency, 2016a, U.S. Department of Energy, 2009). Although the amount of HF fluid could vary by the shale formation and the quality of local water, it has been estimated that a typical well may require 2–10 million gallons of water for HF completion (Harper, 2008, Ernstoff and Ellis, 2013, Yang et al., 2015). As the development of shale gas in the U.S. intensifies, concerns regarding its direct environmental impacts, especially on the consumption and contamination of the surface and ground water, have also increased.
The characteristics of various shale formations can lead to different estimated ultimate recovery (EUR) of shale gas. As the amount of HF water is closely associated with the production of shale gas and liquid, the water consumption factor (WCF) metric has been introduced to reflect the quantity of water directly consumed per energy unit of the final fuel products (Lampert et al., 2016, Clark et al., 2013). However, the EUR is an estimation of the quantity of fossil energy that is recoverable from a well, which could vary due to time, method, and technology of the exploration and estimation. For instance, U.S. Energy Information Administration (EIA) estimated the EUR of the Marcellus play at 0.39 billion cubic feet (BCF)/well (average of 1200 wells) in 2008, which increased to 5.78 BCF/well (average on 2191 wells) in 2012 (U.S. Energy Information Administration, 2016j, Staub, 2015). The US Geological Survey assessed the Jurassic and Cretaceous rocks of the US Gulf Coast back in 2010 and estimated a mean of 70.4 TCF of NG was contained in the Bossier and Haynesville Formations. In an updated 2017 USGS assessment, the same regions were estimated to contain a mean of 304.4 TCF of NG, in addition to 4.0 billion barrels of oil and 1.9 billion barrels of natural gas liquid (U.S. Geological Survey, 2017). Therefore, a sensitivity analysis of the WCFs affected by the EUR variations seems necessary for understanding the overall impact on water depletion by the shale gas pathways.
Argonne National Laboratory has been evaluating the NG fuel pathways with a focus on quantifying the methane leakage of the NG supply chain and its impacts on WTW GHG emissions of NG fuel pathways and NGVs (Burnham et al., 2012, Burnham, 2016, Wang and Huang, 2000). The National Research Council discussed GHG emissions from CNG use primarily based on previous Argonne studies and the Greenhouse gases, Regulated Emissions, and Energy use in Transportation (GREET) model (National Research Council, 2013). Rose et al. (2013) found that significant reductions in life-cycle GHG and criteria air pollutant emissions were obtained by CNG refuse trucks compared to the diesel counterparts based on real-time operational data in British Columbia, Canada. WTW GHG emissions of light-duty, medium-duty, and heavy-duty vehicles powered by NG-derived fuels, including CNG, LNG, electricity, and hydrogen were examined and compared to their gasoline and diesel counterparts, and vehicle fuel efficiency and methane leakage rate of the NG supply chain were found to be major drivers to the relative GHG emission performances of NG-based vehicles (Tong et al., 2015a).
Recently, new data covering previously overlooked emission sources of methane leakage associated with the NG supply chain, such as gas gathering plants, have been collected and provide a new opportunity to examine the GHG emission impacts of the NG industry (Zavala-Araiza et al., 2015; Littlefield et al., 2017). In an effort to reconcile divergent estimates of methane leakage from natural gas production through distribution obtained from top-down and bottom-up measurements, Zavala-Araiza et al. (2015) refined sampling techniques for top-down estimates and accounting methodologies for bottom-up estimates. Zavala-Araiza et al. (2015) found that estimates in the Barnett shale oil and gas play using both approaches converged with a 10% difference. While the methane leakage represented about 1.5% of natural gas production, a small proportion of high-emitters were found responsible for half of total methane emissions. Lately, a synthesis of methane emission data from a series of ground-based field measurements shows that 1.7% of the methane in natural gas is emitted from the production through distribution natural gas supply chain (Littlefield et al., 2017). Extending this natural gas supply chain to fuel production, refueling, and end use, recent findings on CNG and LNG refueling stations and vehicle methane emissions have identified a major information gap on the life-cycle performance of NGVs (Clark et al., 2016). The impact of these station and vehicle emissions is not well known and needs to be assessed on a WTW basis.
One of the original drivers of NGV development has been its ability to have low air pollutant emissions. Diesel HDVs contribute substantially to nitrogen oxides (NOx) and particulate matter (PM) emissions and to the resultant air quality effects of ground-level ozone formation driven by NOx and the adverse health impacts of PM (Nelson et al., 2008, Peretz et al., 2008, Pope, 2004). In 2016, 211 U.S. counties with 119 million people, about 40% of the U.S. population, were in ozone nonattainment (i.e., not meeting the U.S. air quality standards) (U.S. Environmental Protection Agency, 2016b). Twelve counties in California's South Coast and San Joaquin air basins with a total population of 20 million were designated as “extreme” ozone nonattainment areas (U.S. Environmental Protection Agency, 2016b). These areas had ozone values 233% higher (175 ppb) than the previous standard of 75 ppb. In 2015, the U.S. Environmental Protection Agency (EPA) revised the ozone standard to 70 ppb, which results in 241 counties not meeting the standard based on 2012–2014 air quality data.
In 2016, 20 counties with 23 million people were in PM2.5 (PM with aerodynamic diameters less than 2.5 µm) nonattainment areas with respect to the most recent 2012 annual standard. In 2016, additional areas were in non-attainment of previous PM2.5 standards as 35 counties with 13 million people were in nonattainment of the 2006 24-h PM2.5 standard and 13 counties with 3 million people were in nonattainment of the 1997 annual PM2.5 standard (U.S. Environmental Protection Agency, 2016b). In total 68 counties with 39 million people are in PM2.5 non-attainment in 2016.
These situations have led to increasingly stringent standards for NOx and PM emissions, with the EPA tightening the HDV engine standards for both pollutants by ~ 98% from 1988 to 2010. Currently, the EPA and California Air Resources Board (CARB) standards for NOx and PM are 0.2 g/bhp-hr (break horsepower-hour) and 0.01 g/bhp-hr, respectively. Because of severe air quality concerns in California, CARB adopted optional low NOx standards in 2014, with three levels to which engines can be certified: 0.10, 0.05, or 0.02 g/bhp-hr. In 2016, the South Coast Air Quality Management District along with 10 other state and local agencies, petitioned the EPA to revise the EPA HDV NOx standard to be 0.02 g/bhp-hr (South Coast Air Quality Management District, 2016). The petition states that to meet the new 70 ppb ozone standard, these areas will need the lower HDV engine standard for NOx emissions.
The first heavy-duty engines to meet both the EPA/CARB 2010 standards and the CARB optional 0.02 g/bhp-hr NOx standard were natural gas powered 8.9-liter (L) engines, suited for refuse, transit, and freight (below 66,000 pounds gross vehicle weight) applications (Cummins Westport Inc, 2016). However, to meet these standards, NGVs rely on the integration of sophisticated engine controls and aftertreatment devices and not just the inherent qualities of the fuel. While all engines sold today meet the EPA/CARB 2010 standards, to fully understand the relative emissions of NGVs as compared to diesel vehicles, analysis of in-use performance is crucial (Nylund and Koponen, 2012, Miller et al., 2013, Yoon et al., 2013, Carder et al., 2014, Wang et al., 2015b, Quiros et al., 2016, Johnson et al., 2016, Anenberg et al., 2017, Sandhu et al., 2017).
The rapid increase in NG production from shale formations has led to significant interest in its use for transportation. NGVs typically have lower and more stable fuel costs than their conventional counterparts (U.S. Department of Energy, 2017a). However, due to the cost of NG storage tanks, NGVs cost significantly more than gasoline and diesel vehicles. In recent years, the focus on heavy-duty NGVs in lieu of light-duty vehicles has been driven by economics (Rood Werpy et al., 2010; Krupnick, 2011; Deal, 2012; Gao et al., 2013). Many heavy-duty vehicles are large fuel users that are able to pay back the incremental vehicle cost via the use of lower cost NG fuel (Johnson, 2010; Deal, 2012; National Petroleum Council, 2012; Laughlin and Burnham, 2014; Jaffe et al., 2015). The literature has found that the major factors that drive the payback of NGVs versus diesels include incremental vehicle cost, fuel price differential, vehicle usage, and relative fuel efficiency.
Section snippets
Methodology and data
We defined a WTW system boundary to estimate the GHG emissions, freshwater consumption, air pollutant emissions, and costs of NGVs, as shown in Fig. 2. The NGVs we analyzed include a Class 8 LNG combination short-haul truck, a Class 8 CNG transit bus, and a Class 8 CNG refuse truck, in comparison to their diesel counterparts. These NGVs represent the major niche markets that heavy-duty NGVs have entered in the U.S. (Cai et al., 2015b). The functional unit used is one ton of freight cargo moved
WTW water consumption
WTW water consumption of NGVs provides an important angle to evaluate the environmental and sustainable impacts of utilizing NG as a feedstock to produce transportation fuels in a water resource constrained world. When the U.S. average NG mix that consists of 47% of shale gas and 53% of conventional NG is the feedstock for LNG production, the LNG combination short-haul truck has a WTW water consumption of about 0.0044 gallon per ton-mile (gal/ton-mi), assuming that freshwater-free air or
Conclusions
Environmental impacts of NGVs from the perspective of water consumption, GHG emissions, and NOx and PM emissions were evaluated on a WTW basis. Significant reduction in water consumption is found for NGVs compared to their diesel counterparts, despite the variation in water consumption associated with shale gas production in various regions. This provides an opportunity to lessen the water stress caused by production and use of diesel in the HDV sector with the introduction and potential
Acknowledgments
This study was supported by the Vehicle Technologies Office (VTO) of the U.S. Department of Energy's Office of Energy Efficiency and Renewable Energy under Contract DE-AC02-06CH11357. We thank Dennis Smith of the VTO's Clean Cities Program and Kevin Stork of VTO's Fuels and Lubricants Team for their support and guidance.
References (100)
- et al.
Water usage for natural gas production through hydraulic fracturing in the United States from 2008 to 2014
J. Environ. Manag.
(2016) - et al.
Synthesis of recent ground-level methane emission measurements from the U.S. natural gas supply chain
J. Clean. Prod.
(2017) - et al.
Effects of vehicle type and fuel quality on real world toxic emissions from diesel vehicles
Atmos. Environ.
(2008) - et al.
A comparative life cycle assessment of diesel and compressed natural gas powered refuse collection vehicles in a Canadian city
Energy Policy
(2013) - et al.
Methods of dealing with co-products of biofuels in life-cycle analysis and consequent results within the U.S. context
Energy Policy Sustain. Biofuels
(2011) - Argonne National Laboratory, 2016. Summary of expansions, updates, and results in GREET® 2016 Suite of...
- et al.
Impacts and mitigation of excess diesel-related NOx emissions in 11 major vehicle markets
Nature
(2017) - et al.
Suggested reporting parameters for investigations of wastewater from unconventional shale gas extraction
Environ. Sci. Technol.
(2013) - et al.
Methane leaks from North American natural gas systems
Science
(2014) - et al.
Energy intensity and greenhouse gas emissions from tight oil production in the bakken formation
Energy Fuels
(2016)
Life-cycle greenhouse gas emissions of shale gas, natural gas, coal, and petroleum
Environ. Sci. Technol.
Well-to-wheels greenhouse gas emissions of Canadian oil sands products: implications for US Petroleum Fuels
Environ. Sci. Technol.
Life cycle water consumption for shale gas and conventional natural gas
Environ. Sci. Technol.
Pump-to-wheels methane emissions from the heavy-duty transportation sector
Environ. Sci. Technol.
Energy efficiency and greenhouse gas emission intensity of petroleum products at US refineries
Environ. Sci. Technol.
Clearing the waters of the fracking debate
Mich. J. Sustain.
Hydraulic fracturing water use variability in the United States and potential environmental implications
Water Resour. Res.
The Marcellus Shale – an old “new” gas reservoir in Pennsylvania
Pa. Geol.
Water use and management in the Bakken Shale Oil Play in North Dakota
Environ. Sci. Technol.
STEPS White Paper: Exploring the Role of Natural Gas in U.S. Trucking
Business Case for Compressed Natural Gas in Municipal Fleets, NREL/TP-7A2-47919
Ultra-Low NOx Natural Gas Vehicle Evaluation ISL G NZ
Effective permeabilities of abandoned oil and gas wells: analysis of data from Pennsylvania
Environ. Sci. Technol
Identification and characterization of high methane-emitting abandoned oil and gas wells
Proc. Natl. Acad. Sci. USA
Methane emissions estimate from airborne measurements over a western United States natural gas field
Geophys. Res. Lett.
Water footprint of hydraulic fracturing
Environ. Sci. Technol. Lett.
Water quality and quantity impacts of hydraulic fracturing
Curr. Sustain. Energy Rep.
Direct measurements show decreasing methane emissions from natural gas local distribution systems in the United States
Environ. Sci. Technol.
Wells to wheels: water consumption for transportation fuels in the United States
Energy Environ. Sci.
Methane emissions from United States natural gas gathering and processing
Environ. Sci. Technol.
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“This article is part of a Virtual Special Issue entitled ''The Potential of Natural Gas as a Sustainable Transportation Fuel.”.